Anhydride-cured epoxy resins are made up of an epoxy resin component [e.g. a diglycidyl ether of bisphenol A (DGEBA)], an anhydride cross-linking agent [e.g. phthalic anhydride (PA)], and a catalyst (e.g. benzyldimethylamine (BDMA)]. Diluents or fillers are also often added for economic or processing reasons. The resin, curing agent and catalyst are mixed and "cured" usually at elevated temperatures. Such a plastic typically has a tensile strength of about 70 MPa, a tensile modulus of about 2800 MPa, an elongation-to-break of 1-6%, and a glass transition temperature (T.sub.g) of about 140.degree. C. All these values will vary somewhat with the stoichiometry of the mixture and the curing conditions (see for example, H. Lee and K. Neville, "Handbook of Epoxy Resins", McGraw-Hill, New York, 1967). Anhydride-cured epoxy resins have excellent electrical properties, good thermal stability and good chemical resistance, but are generally regarded as being brittle, which limits their use in structural applications where high modulus and strength as well as good impact resistance are required. Considerable effort has been expended to improve on these properties, but no completely satisfactory solution currently exists because improvement in one property has been gained only at the expense of others (e.g. solvent resistance, T.sub.g, ductility, modulus, processability, cost).
Many anhydride curing agents or hardeners have been used in epoxy resin systems, in particular multi-ring alicyclic anhydrides such as methyl-bicyclohept-5-ene-2,3-dicarboxylic acid anhydride, also known by the trademark Nadic methyl anhydride (NMA) or aromatic anhydrides such as phthalic anhydride (PA). In some cases, organic acids are used in conjunction with anhydrides to reduce cure times and improve impact properties. In U.S. Pat. No. 2,890,210, Phillips et al, 1959, the use of a carboxyl terminated polyester is described as a co-curing agent with an anhydride for epoxy resins. May (SPE Transactions 3, 251-9, 1963) describes the replacement of some of the PA curing agent for DGEBA resin with phthalic acid, and its effect on the mechanical properties and heat deflection temperature of the plastic.
Antiplasticisation in cross-linked epoxy resin systems has been described in the literature. For instance, N. Hata et al (J. Appl. Polym. Sci. 17, 2173-81, 1973) describe pentachlorobiphenyl as an effective anti-plasticiser, with dibutylphthalate and 2,2 bis[4-(2-hydroxy-3-phenoxyl-propoxy)phenyl]propane as less effective antiplasticisers. There evidently has been little recent interest in this, possibly because materials such as pentachlorobiphenyl are environmentally objectionable.
We have found an alternative method of improving the mechanical properties of anhydride-cured epoxy resins by adding reaction products which act as fortifiers to conventional epoxy resin+anhydride curing agent formulations.